Fabrication of metal organic framework materials using a layer-by-layer spin coating approach

ABSTRACT

Embodiments describe a method of depositing an MOF, including depositing a metal solution onto a substrate, spinning the substrate sufficient to spread the metal solution, depositing an organic ligand solution onto the substrate and spinning the substrate sufficient to spread the organic ligand solution and form a MOF layer.

CROSS REFERENCE TO RELATED APPLICATION

This application claims benefit of U.S. Provisional Application No.62/049,335, filed on 11 Sep. 2014 and which application is incorporatedherein by reference. A claim of priority is made.

BACKGROUND

Many applications including sensing and membranes rely on efficient andreliable methods for fabricating porous materials thin films. Generally,porous thin films can be fabricated by attaching porous material to asurface, such as a functionalized substrate. It is critical that porousthin film fabrication methods be robust, and capable of generatinghomogeneous films with well-defined thicknesses. In some cases, a highdegree of orientation can be beneficial. Metal-organic frameworks (MOFs)are a new class of crystalline porous materials, which are very wellsuited for use as surface-modifying coatings. The adaptation of specificfunctions to MOF thin films can be achieved either by loading depositedMOF with functional molecules or by through a post-functionalizingmodification of the MOF-constituents.

Liquid phase epitaxy (LPE) fabrication processes (or layer-by layermethods) can be used to obtain crystalline, highly oriented MOFs layerson substrates. Substrates can include modified Au-substrates, alsoreferred to as SURMOFs. A major drawback of this LPE method, however, isthat the sequential deposition process is time consuming and tedious.For example, an LPE method can require 400 separate immersion cycles tofabricate a film 100 layers (i.e., ˜100 nm) thick. Such a process cantake approximately 3 days, and further consume large quantities ofchemicals and solvents. These drawbacks preclude the use of MOFs fromapplications which require thicker and/or mechanically strong layers,such as membranes, storage, and small molecule separation (e.g., gasphase chromatography, liquid phase chromatography) where a thickness ofat least a 1 μm can be required.

Spray methods can be used to fabricate MOFs at speeds which are twoorders of magnitudes faster than LPE fabrication processes. The spraymethod utilizes a nozzle system which deposits the reactant's solutionsand solvents required for the MOF thin film growth onto a targetedsurface (i.e., a substrate) in a form of aerosol. The aerosol droplets,having sizes down to about 10 μm, impinge on the substrate and coat thesurface with a thin film of the desired reactant. The reactant, either ametal precursor or an organic ligand, deposits at the solid/liquidinterface in a similar fashion as with the LPE process. The coatedsurface is next sprayed with solvent to remove unreacted material.However, nozzle limitations make it difficult to apply a homogenouscoating, and the process consumes a large amount of chemicals andsolvents, thereby making the process prohibitively inefficient at largerscales.

SUMMARY

Embodiments describe a method of depositing an MOF, including depositinga metal solution onto a substrate, spinning the substrate sufficient tospread the metal solution, depositing an organic ligand solution ontothe substrate and spinning the substrate sufficient to spread theorganic ligand solution and form a MOF layer.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 illustrates a block flow diagram of a method of depositing alayer of MOF on a substrate, according to one or more embodiments.

FIG. 2A illustrates out-of-plane X-ray diffraction data of aCu₂(bdc)₂.H₂O SURMOF grown on a COOH— terminated SAM after differentcycles, according to one or more embodiments.

FIG. 2B illustrates a plot of a Cu₂(bdc)₂.H₂O SURMOF thickness pergrowth cycle, according to one or more embodiments.

FIG. 2C illustrates out-of-plane X-ray diffraction data of aZn₂(bdc).xH₂O SURMOF grown on a COOH terminated Au-substrate, accordingto one or more embodiments.

FIG. 3 illustrates the results of a gas separation study for a ZIF-8 MOFmembrane, according to one or more embodiments.

FIG. 4 illustrates a data for NH₃ sensing capabilities of aCu₂(bdc)₂.xH₂O MOF on QCM substrate, according to one or moreembodiments.

DETAILED DESCRIPTION

Described herein are novel spin coating methods for fabricating highlyoriented and crystalline coatings of highly porous metal-organicframeworks (MOFs) on various solid substrates. The methods are capableof producing homogeneous MOF films with controllable thicknesses, andare based on adapting LPE synthesis to spin coating. Such spin coatingmethods not only can provide significantly higher output as compared toLPE and dipping processes, but also dramatically decrease theconsumption of chemicals and solvents, which make them more economicallyfeasible for industrial application. Through applying this new approach,it is possible to fabricate thick (μm) layers of several types of MOFs,including Cu(bdc)₂.xH₂O, Zn(bdc)₂.xH₂O, HKUST-1 and ZIF-8, on varioussubstrates like functionalized Au and aluminum oxide substrates.

Metal organic framework materials (MOFs) are crystalline materialscomposed of both inorganic and organic components in a porous networkedstructure. Metal organic framework materials exhibit exceptionally highspecific surface area, in addition to tunable pore size andfunctionality, which make them attractive in many applications,including gas storage, gas separation, catalysis, drug delivery,light-emitting devices, and sensing.

MOFs can be applied in gas storage for hydrogen and hydrocarbons, gasseparation, gas sensors and catalysis. MOFs can include metal cationsM^(n+) (n=2, 3) and functionalized polytopic organic ligands. MOFs havecavities and/or channels readily available for adsorption of guestmolecules, embedding of nano-clusters or more generally for anchoringvarious functional species to match the pore size in the scaffold. Onetheme of this chemistry is the rational design and precise control ofthe formation of the respective network.

Metal-organic frameworks (MOFs) can be made by linking inorganic andorganic units by strong bonds. The flexibility with which theconstituents' geometry, size, and functionality can be varied can leadto many different MOFs. The organic units can be ditopic, polytopicorganic carboxylates (and other similar negatively charged molecules),which, when linked to metal-containing units, can yield architecturallyrobust crystalline MOF structures with a porosity of greater than 50% ofthe MOF crystal volume. The surface area values of such MOFs can rangefrom 1000 to 10,000 m²/g, thus exceeding those of traditional porousmaterials such as zeolites and carbons. To date, MOFs with permanentporosity are more extensive in their variety and multiplicity than manyother classes of porous materials. These aspects have made MOFs idealcandidates for storage of fuels (hydrogen and methane), capture ofcarbon dioxide, and catalysis applications, to mention a few.

The use of MOFs as powder materials is evident, but the integration ofMOFs as novel building blocks and functional units for bottom-upnanotechnology requires precise control over the process ofcrystallization of MOFs on surfaces. Smart membranes, catalyticcoatings, chemical sensors, and many other related nanotechnologyapplications depend on the production of controlled thin films andcoatings with well-defined porosity, chemical composition, and tunablefunctionality. Zeolites, MOFs, organic polymers, metal oxides andactivated carbon can be used. The growth of thin MOF films can use anin-situ crystallization method on surfaces, in particular of MOF-5,HKUST-1 and many other MOFs. Oriented MOF thin films can be anchoredfrom mother liquor solution as a function of the surface functionalgroups of self-assembled monolayers (SAMs) (COOH and OH, respectively)for different types of MOFs.

Layer-by-layer (or Liquid phase epitaxy) method can be used to grow thinMOF layers. For example, HKUST-1 MOF can be grown using LBL method onmetal surfaces functionalized with SAMs, specifically to steer andcontrol the growth orientation.

The spin-coating approach offers a platform for depositing uniform thinfilms to substrates that can be tuned easily and used. The method can beeasily automated using the available spin coating commercial devices.Inorganic, organic components and solvents are tunable, which permitsfacile alteration of MOF layers and functionality. It is also possibleto scale up on different substrate sizes and there is no limitation forsubstrate type, such as metals, metal oxides, polymers, etc.

A method 100 for depositing a layer of MOF 110 on a substrate 105 isillustrated in FIG. 1. Method 100 can comprise depositing 110 a metalsolution onto a substrate, spinning 115 the substrate to spread themetal solution, depositing 120 an organic ligand solution on thesubstrate, and spinning 125 the substrate to spread the organic ligandsolution. The metal solution can comprise a metal ion, such as fromtransition metals and alkaline earth metals. The metal ion can comprisea divalent metal cation. The divalent metal cation (M²⁺) can compriseNi, Cu, Zn, Mn, Cd and Co. The metal ion can comprise a trivalent metalcation. The trivalent metal cation (M³⁺) can comprise Fe, and Al, forexample. The organic ligand solution can include polytopic ligands, suchas ditopic ligands. Examples include terephthalic acid and itsderivatives, such as amino, nitro, and hydroxyl. Other examples includedabco, bipyridine, and pyrazine. Further examples include trivalentembodiments, such as trimesic acid and many other polytopic ligands. Thesubstrate can include alumina, titanium oxide, polymer, copolymer,carbon, metal, or metal oxides.

Depositing 110 and depositing 120 can be accomplished using a syringe,such as a micro syringe, to deposit a drop, such as a micro-drop. Asmall amount of coating material can be deposited on the center of thesubstrate, which is either spinning at low speed (such as around 100rpm) or not spinning at all. The substrate can then be rotated at highspeed (such as 500-6000 rpm) in order to spread the coating material bycentrifugal force. A machine, such as a spin coater, can be used forspin coating.

Spinning 115 and spinning 125 can occur for an amount of time. An amountof time can be determined based on a time sufficient to spread a metalsolution or an organic ligand solution thoroughly across the substrate.Spreading can occur by centrifugal force during spinning An amount oftime can be determined based on a time sufficient to for a metalsolution to react with an organic ligand solution. A MOF can form on thesubstrate as a result of subsequent deposition of a metal solution andan organic ligand solution. One cycle 150 includes depositing 110,spinning 115, depositing 120, and spinning 125. Depositing 110 andspinning 115 can occur before or after depositing 120 and spinning 125.Multiple cycles can be repeated to generate a thicker MOF film. Spinningcan be continued while the fluid spins off the edges of the substrate,until the desired thickness of the film is achieved. The applied solventcan be usually volatile, and simultaneously evaporates. So, the higherthe angular speed of spinning, the thinner the film. The thickness ofthe film can also depend on the viscosity and concentration of thesolution and the solvent, which can range from about 1-2 nm to about300-400 nm.

Method 100 can optionally include one or more washing steps. A washingstep can comprise depositing a solvent onto the substrate after spinning115 or spinning 125. Depositing a solvent can comprise depositing a dropof solvent. Examples of solvents include ethanol, methanol, DMF andothers. Solvent can optionally be deposited to remove unreacted orunadhered metal solution or organic ligand solution from the substrate.The amount of solvent deposited can depend on the amount of metalsolution or organic ligand solution that needs to be removed. With afully automated spin coating machine, a micro-drop of a metal solutionusing a micro syringe is placed on the substrate for a certain time,which is rotated at high speed in order to spread the fluid bycentrifugal force.

EXAMPLE 1 Spin Coating Fabrication of M₂(CH₃COO)₄.H₂O (M=Cu, Zn) MOFs

Cu₂(bdc)₂.xH₂O and Zn₂(bdc)₂.xH₂O MOFs were grown on gold substrates(200-nm Au/2-nm Ti evaporated on Si wafers) that were at firstfunctionalized by self-assembled monolayers SAMs of 16mercaptohexadecanoic acid MHDA. These substrates were then placed on avacuum chuck and subsequently spin coated with 50 μl of a 1 mMM₂(CH₃COO)₄.H₂O (M=Cu, Zn) ethanol solution for 5 seconds and then with50 μl of a 0.1 mM BDC solution for 5 seconds at room temperature.Critical parameters of the spin coating procedure are spin coating speed(500 rpm), spinning time and injection volume. Typical values of theseparameters are 800 rpm, 5 seconds, and 50 μl, respectively. Between eachstep the substrates were washed with solvent. After a carefuloptimization of the time, concentration, and spinning speed, it wasfound that the time needed to deposit 100 cycles of thin film took only50 minutes, as compared to the 72 hours required by the conventionallayer-by-layer process and 30 minutes compared to the spray method.While the spin coating method produced a film of equal thickness to afilm produced by a spray method over a number of cycles (i.e. 200 nm for10 cycles), the spin coating method used less solution and furtheroffers the possibility to easily coat larger surfaces. The spin coatedfilms were found to be very stable upon drying and removing of thesolvent—both on the macro and micro scale and no kind of delaminationwas observed.

FIG. 2A illustrates out-of-plane X-ray diffraction data(background-corrected) of the Cu₂(bdc)₂.H₂O SURMOF grown on a COOH—terminated SAM after different cycles. Calculated data for Cu₂(bdc)₂.H₂Ois shown for comparison. The diffraction peaks reveal the presence of acrystalline and highly oriented Cu(bdc)₂.xH₂O film. Interestingly,although the time to grow this MOF thin film is two orders of magnitudefaster than with the conventional LPE-method, the degree of orientationand ordering is comparable.

FIG. 2B illustrates a plot of the Cu₂(bdc)₂.H₂O SURMOF thickness pergrowth cycle. This thickness analysis shows the increase ofCu(bdc)₂.xH₂O film thickness per deposition cycle, which is much largerthan film thickness achieved by LPE deposition processes. These resultsconfirm that the spin coating methods provided herein have a much fastergrowth rate per cycle than LPE methods.

FIG. 2C illustrates Out-of-plane X-ray diffraction data(background-corrected) of an Zn₂(bdc).xH₂O SURMOF grown on a COOHterminated Au-substrate after 10 cycles in additional to calculatedpatterns.

The spin coating method was also used to grow MOF thin films onprepatterned substrates, proving the method to be highly selective. SEMimages confirmed the deposition of MOF thin films only onCOOH-terminated substrate regions, with negligible MOF deposition onCH₃-terminated substrate regions. Before deposition, the SAMs had beenlaterally patterned via micro-contact printing (μCP) method.

EXAMPLE 2 Spin Coating Fabrication of ZIF-8 MOFs Membrane

ZIF-8 MOFs were grown on highly porous Al₂O₃ (Cobra Technologies BV)substrates. The support was first washed with water and ethanol and thendried at 150° C. to remove any contaminants from the surface. Thesesubstrates were then placed on a vacuum chuck and subsequently spincoated with 50 μl of a 2 mM of Zn(NO₃)₂*6H₂O methanol solution for 5seconds and then with a 50 μl of a 2 mM of 2-Methylimidazole methanolsolution for 8 seconds at room temperature. The spin coating procedureused for fabricating the Cu₂(bdc)₂.H₂O MOF in Example 1 was followed.

In this example, the ZIF-8 the spin coating method was found to be muchmore efficient than the conventional LPE method, based on 200 growthcycles yielding a thickness of 5 μm in about 1.5 hours versus the LPEmethod which required 150 growth cycles to yield a thickness of 0.6micrometer in 24 hours.

FIG. 3 illustrates the results of a gas separation study, wherein thepermeation of H₂ and CO₂ were tested against the ZIF-8 MOF membrane. ACO₂/H₂ 20/80 mixture was tested using a variable pressures-continuouspermeate composition analysis technique at 308 K with 2.0 bar as a feedpressure. The variable pressure-continuous permeate composition analysistechnique was used to test the gas mixture permeation for the membrane.The permeate gas composition is monitored continuously until theoccurrence of the stead state. After an activation process of themembrane with He gas, the binary gas mixture with composition a_(up),b_(p) of interest is applied upstream with a maintained flux at 40-50cc/min while monitoring the composition of the permeate downstreamcomposition a_(down), b_(down). The system is considered in a steadystate when no change in the signal of the MS is observed

The mixture gas permeation results show that the ZIF-8 thin film shows aselectivity of about 4.6 in favor of H₂.

EXAMPLE 3 Spin Coating Fabrication of HKUST-1 MOFs

The HKUST-1 was grown on activated aluminum substrates (same as ZIF-8).These substrates were then placed on a vacuum chuck and subsequentlyspin coated with 50 μl of a 0.5 mM of Cu₂(CH₃COO)₄.H₂O ethanol solutionfor 5 seconds and then with a 50 μl of a 0.2 mM of1,3,5-benzenetricarboxylic acid ethanol solution for 10 seconds at roomtemperature. The spin coating procedure for fabricating theCu₂(bdc)₂.H₂O MOF in Example 1 was followed.

EXAMPLE 4 Spin Coating Fabrication of M₂(CH₃COO)₄.H₂O on QCM Substrates

Quartz crystal microbalance (QCM) substrates were coated withCu₂(bdc)₂.xH₂O MOF using the LPE spin coating approach to test them forsensing application. The Cu₂(bdc)₂. x H₂O SURMOF was spin coated over aCOOH functionalized QCM substrate using the spin coating techniques asdescribed in the previous examples. The coated Cu₂(bdc)₂.xH₂O MOF on QCMsubstrate was tested for an ability to sense NH₃ in humid conditions.The mass uptake for the pure H₂O feed was measured as a reference. Asillustrated in FIG. 4, a slow mass uptake is observed. However, in caseof the 1% NH₃ in water solution, a very rapid uptake was observed on theQCM, which indicates the high selectivity of this MOF for the NH₃ gas.Despite its similar size, NH₃ showed much faster uptake due to itscapability to coordinate stronger to the Cu²⁺ atoms of theCu₂(bdc)₂.xH₂O MOF framework.

Embodiments of the present invention include, at least the following:

-   Embodiment 1. A method of depositing a MOF, comprising:    -   depositing a metal solution onto a substrate;    -   spinning the substrate, sufficient to spread the metal solution;    -   depositing an organic ligand solution onto the substrate; and    -   spinning the substrate, sufficient to spread the organic ligand        solution and form a MOF layer.-   2. The method of embodiment 1, wherein the metal solution comprises    a metal ion or cluster.-   3. The method of embodiment 2, wherein the metal ion comprises a    divalent metal cation, trivalent metal cation, clusters of a    divalent or trivalent metal ions, or combinations thereof.-   4. The method of embodiment 1, wherein the organic ligand solution    comprises polytypic ligands.-   5. The method of embodiment 1, wherein the organic ligand solution    comprises one or more of terephthalic acidacid, trimesic acid,    dabco, bipyridine, and pyrazine.-   6. The method of embodiment 1, wherein the substrate comprises    alumina, titanium oxide, polymer, copolymer, carbon, metal, or a    metal oxide.-   7. The method of embodiment 1, wherein depositing comprises using a    syringe to deposit one or more drops.-   8. The method of embodiment 1, wherein spinning comprises two or    more constant speeds.-   9. The method of embodiment 1, further comprising one or more cycles    of depositing a metal solution and spinning-   10. The method of embodiment 1, further comprising one more cycles    of depositing an organic ligand solution and spinning.-   11. The method of embodiment 1, further comprising washing after    spinning.-   12. The method of embodiment 11, wherein washing comprises    contacting with a solvent.-   13. The method of embodiment 1, wherein the organic ligand solution    comprises two or more different polytopic ligands.-   14. The method of embodiment 1, further comprising controlling the    thin film thickness and orientation.

What is claimed is:
 1. A method of depositing a MOF, comprising:depositing a metal solution onto a substrate; spinning the substrate,sufficient to spread the metal solution; depositing an organic ligandsolution onto the substrate; and spinning the substrate, sufficient tospread the organic ligand solution and form a MOF layer.
 2. The methodof claim 1, wherein the metal solution comprises a metal ion or cluster.3. The method of claim 2, wherein the metal ion comprises a divalentmetal cation, trivalent metal cation, clusters of a divalent ortrivalent metal ions, or combinations thereof.
 4. The method of claim 1,wherein the organic ligand solution comprises polytypic ligands.
 5. Themethod of claim 1, wherein the organic ligand solution comprises one ormore of terephthalic acidacid, trimesic acid, dabco, bipyridine, andpyrazine.
 6. The method of claim 1, wherein the substrate comprisesalumina, titanium oxide, polymer, copolymer, carbon, metal, or a metaloxide.
 7. The method of claim 1, wherein depositing comprises using asyringe to deposit one or more drops.
 8. The method of claim 1, whereinspinning comprises two or more constant speeds.
 9. The method of claim1, further comprising one or more cycles of depositing a metal solutionand spinning
 10. The method of claim 1, further comprising one morecycles of depositing an organic ligand solution and spinning
 11. Themethod of claim 1, further comprising washing after spinning
 12. Themethod of claim 11, wherein washing comprises contacting with a solvent.13. The method of claim 1, wherein the organic ligand solution comprisestwo or more different polytopic ligands.
 14. The method of claim 1,further comprising controlling the thin film thickness and orientation.